A simplified diagrammatic representation of a disk drive, generally designated as 10, is illustrated in FIG. 1. The disk drive 10 includes a disk stack 12 (illustrated as a single disk in FIG. 1) that is rotated by a spindle motor 14. The spindle motor 14 is mounted to a base plate 16. An actuator arm assembly 18 is also mounted to the base plate 16.
The actuator arm assembly 18 includes a transducer 20 (or head) mounted to a flexure arm 22 which is attached to an actuator arm 24 that can rotate about a pivot bearing assembly 26. The actuator arm assembly 18 also includes a voice coil motor 28 which moves the transducer 20 relative to the disk 12. The spin motor 14, and actuator arm assembly 18 are coupled to a number of electronic circuits 30 mounted to a printed circuit board 32. The electronic circuits 30 typically include a digital signal processor (DSP), a microprocessor-based controller and a random access memory (RAM) device.
Referring now to the illustration of FIG. 2, the disk stack 12 typically includes a plurality of disks 34, each of which may have a pair of disk surfaces 36, 36. The disks 34 are mounted on a cylindrical shaft and are designed to rotate about axis 38. The spindle motor 14 as mentioned above, rotates the disk stack 12.
Referring now to the illustration of FIGS. 1 and 3, the actuator arm assembly 18 includes a plurality of the transducers 20, each of which correspond to one of the disk surfaces 36. Each transducer 20 is mounted to a corresponding flexure arm 22 which is attached to a corresponding portion of the actuator arm 24 that can rotate about the pivot bearing assembly 26. The VCM 28 operates to move the actuator arm 24, and thus moves the transducers 20 relative to their respective disk surfaces 36.
Although the disk stack 12 is illustrated having a plurality of disks 34, it may instead contain a single disk 34, with the actuator arm assembly 18 having a corresponding single actuator arm 24.
FIG. 4 further illustrates one of the disks 34. Data is stored on the disk 34 within a number of concentric tracks 40 (or cylinders). Each track is divided into a plurality of radially extending spokes 42 on the disk 34. Each spoke 42 is further divided into a servo spoke 44 and a data spoke 46. The servo spokes 44 of the disk 34 are used to, among other things, accurately position the transducer 20 so that data can be properly written onto and read from the disk 34. The data spokes 46 are where non-servo related data (i.e., user data) is stored and retrieved. Such data, upon proper conditions, may be overwritten.
FIG. 5 illustrates portions of data tracks 47 in two adjacent radial spokes 42 (FIG. 4) labeled m and m+1 on the disk 34, which are drawn in a straight, rather than arcuate, fashion for ease of depiction. To accurately write data to and read data from the data tracks 47 of the disk 34, it is desirable to maintain the transducer 20 in a relatively fixed position with respect to a given data track's centerline 48 during each of the writing and reading procedures (called a track following operation). Three data tracks 47 at radial locations labeled n−1 through n+1, including their corresponding centerlines 48, are shown in FIG. 5.
To assist in controlling the position of the transducer 20 relative to the data track centerline 48, the servo spokes 44 can contain a servo preamble 49 and servo burst patterns 50. The servo preamble 49 can include a write/read (W/R) recovery field, an automatic gain control (AGC) field, a synchronization field, a spoke (sector) number field, and/or a cylinder number field. For purposes of illustration only, the width of the servo burst patterns 50 have been exaggerated relative the width of the servo preamble 49. Fields of a servo spoke are illustrated in U.S. Pat. No. 6,256,160, which is incorporated herein by reference. Unlike information in the data spokes 46, the servo spokes 44 should not be overwritten or erased during normal operation of the disk drive 10.
The W/R recovery field can be used by the disk drive 10 to transition from writing data to a previous data track 47 to reading information in the present servo spoke 44. The AGC field can be used to set a gain of the read/write channel of the disk drive 10. The synchronization field can be used to synchronize a clock so that spoke (sector) and cylinder number fields can be read, and so that the servo burst patterns 50 can be located. The spoke number field can be indicative of the circumferential position of the servo region with respect to the disk 34. The cylinder number field can be indicative of a radial location of the servo region.
The servo burst patterns 50 can include one or more groups of servo bursts, as is well-known in the art. A servo burst pattern 50 that includes first, second, third and fourth servo bursts A, B, C and D, respectively, are shown in FIG. 5. The servo bursts A, B, C, D are accurately positioned relative to each other.
The transducer 20 is positioned relative to a data track 47 (i.e., during a track following operation) based on the servo burst patterns 50 which it reads as it crosses the servo spokes 44, one at a time. The servo burst patterns 50 are used to, among other things, generate a position error signal (PES) as a function of the misalignment between the transducer 20 and a desired position relative to the data track centerline 48. As is well-known in the art, the PES signals are input to a servo control loop (within the electronic circuits 30) which performs calculations and outputs a servo compensation signal which controls the VCM 28 to, ideally, place the transducer 20 at the desired position relative to the data track centerline 48.
A servo track writer (STW) is used to write the servo spokes 44, including their servo burst patterns 50, onto the surface(s) 36 of the disks 34 during the manufacturing process. The STW controls the transducers 20 corresponding to each disk surface 36 of the disks 34 to write the servo regions 44. In order to precisely write the servo burst patterns 50 at desired locations on the disks 34, the STW directs each transducer 20 to write in small steps, with each step having a pitch (i.e., servo track pitch 74 as shown in FIG. 5). FIG. 5 illustrates the relationship between the servo track width 75 and the servo track pitch 74 of the servo burst patterns 50 for a conventional disk drive system.
As used herein, the term “pitch” is the radial distance between centers of adjacent regions on the surfaces 36 of the disks 34. For example, the servo track pitch 74 is the distance between the centers of radially adjacent servo bursts (illustrated between servo bursts C and D). In contrast, the term “width” is the radial distance from one end to the other end of a single region. For example, the servo track width 75 is the width from one end to another end of a single servo burst (illustrated for servo burst D).
For servo spokes 44, the servo track pitch 74 is generally the same as the servo track width 75. For data spokes 46, the data track pitch 76 is generally different from the data track width 78 due to, for example, the presence of erase bands which are typically formed between radially adjacent data tracks 47. For purposes of illustration only, the data track width 78 and the servo track width 75 have been shown to be about the same. However, it is to be understood that their relative sizes can be different, and that the servo track width 75 is generally about ⅔ of the data track width 78.
As shown in FIG. 5, the centerline 48 of the data tracks 47 can be aligned along a centerline of groups of the servo burst patterns 50. When the data track pitch 78 and the servo track pitch 74 are constant across the disk 34, the centerline 48 of the data tracks 47 can remain aligned with a centerline of the groups of servo burst patterns 50. The centerline 48 of the data tracks 47 may, for example, then be aligned as shown in FIG. 5, or between servo bursts A and B. However, in some disk drives the data track pitch 76 can vary radially across the disk 34, such that the alignment of the data tracks 47 and the servo burst patterns 50 varies radially across the disk 34. Referring now to FIGS. 5 and 6, the effect of radial variation in data track pitch on the offset between data tracks 47 and servo burst patterns 50 is illustrated.
FIG. 6 illustrates portions of data tracks 47 at three radial locations labeled track p−1 to track p+1, which are in the same adjacent spokes 42 labeled m and m+1 as shown in FIG. 5. The data tracks 47 have a data track pitch 600 that is different from the data track pitch 76 shown in FIG. 5 and, consequently, the centerlines 48 of the data tracks 47 shown in FIG. 6 are offset by an amount 62 from reference centerlines 60 of the servo bursts A, B, C, D. The data tracks 47 also have a data track width 602 that is generally the same as, but can be different from, the data track width 78 shown in FIG. 5.
Accordingly, as shown by FIGS. 5 and 6, the alignment between the data tracks 47 and the servo bursts A, B, C, D varies radially across the disk 34. The PES that is generated from the servo bursts A, B, C, D can have a gain error that varies based on radial location of the transducer 20 within the servo bursts A, B, C, D. Accordingly, some of the tracks 47, such as tracks n−1 to n+1, may have less gain error than other tracks 47, such as tracks p−1 to p+1, because of the alignment of the track centerlines 48 and the servo bursts A, B, C, D. Consequently, the transducer 20 may be positioned more accurately relative to some of the tracks 47 than other of the tracks 47.